Advertisement posted in the New York newspapers by the Germans.

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The sinking of the Lusitania- May 7th, 1915

Britain used it's powerful navy to blockade Germany. Germany resorted to submarine warfare with their "untersee shiffs" or "U-boats".
In response to the blockade, the Germans had warned about using "unrestricted submarine warfare" against ships coming to Great Britain. See a warning and videos here: Warning!

On May 7th, she was off the coast of Ireland when she was attacked by a German submarine, the "U-20" and sunk with one torpedo.
She sank in about 18 minutes and while going under, a secondary explosion was heard down in the hull leading to speculation.....
Over 2000 on board, around 1200 died including 128 Americans.
The sinking was used by the Allies as propaganda to inflame world opinion about the Germans.
The Germans produced a Lusitania medalfor the sailors in the U-20 for sinking this famous ship, angering people even more.
Conspiricies abound as to whether the ship was not just innocently carrying civilians, but maybe had weaponry hidden in its cargo, which could account for the large "secondary" explosion.